Induced radiative effects such as Raman and fluorescence detection have become extremely useful tools in materials investigation. In a typical optical arrangement used in such an application, a source of excitation energy, typically a beam from a laser, is directed toward a sample to be characterized, and the light emitted by the sample is collected along a collection path, detected, and analyzed using a spectrograph.
Depending upon the configuration, it is often advantageous to combine or fold the excitation energy into the collection path, thereby enabling common optics to be utilized in conjunction with focusing the excitation onto, or into, the sample, and collecting or collimating the spectrum received therefrom. Such a configuration is commonly used in a Raman or fluorescence microscope arrangement, for example.
Heretofore, a holographically derived narrowband filter or notch filter has been utilized as a beam combiner in probes of this type, that is, for the purpose of folding the excitation energy into the collection path. In a Raman microprobe application, for instance, a holographic beamsplitter in the form of a notch filter is now used to replace the dielectric elements previously employed for this purpose. The narrowband filter is typically designed to operate at an acute angle of incidence, and delivers in excess of 90 percent of the incident excitation energy through the objective optic, while transmitting in excess of 80 percent of the wavelength-shifted light emitted by the sample in the direction toward the detector. Various probe head configurations of this type are set forth in commonly assigned U.S. Pat. No. 5,377,004, the content of which is also incorporated herein by reference.
Using holography, highly efficient notch filters may be fabricated as volume-phase optical elements, which comprise recorded fringes of periodically varying refractive index (RI) at least several wavelengths deep in the material. Such structures may be conformal, with the fringes being substantially parallel to the surface of the material, to create a volume reflection optical element, or substantially perpendicular or tilted with respect to the surface of the material, to realize a transmissive device.
One advantage of using a notch filter in a beam-combining spectroscopic application, is that notch filters can now be fabricated with extremely narrowband properties, resulting in a very precise introduction of the excitation into the combined path as a function of laser wavelength. However, being so accurate, beam combiners of this type, including non-holographic reflective dielectric or metallic mirrors, demand extreme physical alignment within a very narrow range of physical tolerance. In addition, particularly in the case of holograms, thicker materials are used to realize narrow notch properties, resulting in background fluorescence and other undesirable spurious scatter caused by the excitation of internal structures.